The present invention relates to a method and apparatus for controlling the temperature during the direct reduction of iron. More particularly, the present invention relates a method and apparatus which controls the temperature uniformity of the center portion of the iron burden in a direct reduction shaft furnace thereby allowing a higher amount of hydrocarbons to be used throughout the reduction process.
The production of direct reduced iron in both hot and cold discharge plants occurs in a vertical shaft furnace and involves reduction of iron ore or iron oxide as it moves downwardly in a reduction zone of a vertical shaft furnace through which is passed a suitable hot reducing gas, known as bustle gas. Bustle gas, which is principally composed of carbon monoxide and hydrogen, is introduced to the shaft furnace at temperatures in the range of about 700xc2x0 C. to about 1100xc2x0 C. The ore is charged at the top of the furnace and caused to flow downwardly through the reduction zone wherein it is reduced by heated reducing gas which flows upwardly through the furnace, after which the reduced ore flows into and downwardly through the transition zone to be carburized if desired. For cool discharge plants, after passing through the reduction zone, the ore is cooled in a cooling zone through which is passed a gaseous coolant at a temperature below about 200xc2x0 C. Typically, in a cool discharge furnace, both the reducing gas and cooling gas are re-circulated, optionally in closed loops, to which streams of fresh (i.e. xe2x80x9cmake-upxe2x80x9d) reducing gas are added and from which streams of spent gas are removed.
The reducing gas being fed to the reduction zone of the furnace is typically at an elevated temperature, which is required by reaction kinetics. The reducing gas is caused to contact the downwardly moving iron ore to reduce the iron oxides therein according to the following basic reactions:
3Fe2O3+H2/COxe2x86x922Fe3O4+H2O/CO2xe2x80x83xe2x80x83(1)
Fe3O4+H2/COxe2x86x923FeO+H2O/CO2xe2x80x83xe2x80x83(2)
FeO+H2/COxe2x86x92Fe+H2O/CO2xe2x80x83xe2x80x83(3)
In the production of direct reduced iron (DRI), it is desirable to increase the product carburization and to increase in-situ reforming in the lower portion of both hot and cold direct reduction furnaces by injecting hydrocarbons. This is a proven means to increase the productivity of direct reduction furnaces without adding new equipment to increase reducing gas capacity. This is also a proven means to increase product carbon. The hydrocarbons react with the hot DRI, depositing carbon and liberating hydrogen gas. However, the reaction of the hydrocarbons to form carbon and hydrogen is endothermic. Thus, the newly formed cool hydrogen gas flows upward through the center of the furnace (called upflow), cooling the descending iron material. Because of temperature considerations, the amount of hydrocarbons that can be added to the lower portion of the furnace is limited by either low center bed temperature or low product discharge temperature.
As more hydrocarbons are added to the lower portion of the furnace, cooled hydrogen gas is produced which rises into the reduction zone and the center bed temperature decreases, thus reducing reaction kinetics. At a temperature of about 625-650xc2x0 C., the average product metallization begins to drop because the material in the center of the furnace is not properly reduced/metallized. Also, in hot discharge furnaces, product discharge temperature must be maintained above approximately 700xc2x0 C. for proper subsequent briquetting. For hot transport applications, higher discharge temperature of the DRI makes more sensible heat available in the melter, thus reducing the power required for melting. As hydrocarbons are added to the lower portion of a hot discharge furnace, it is possible that the average product temperature will be below 700xc2x0 C. before the center bed temperature reaches the point that metallization drops significantly.
To date, several techniques have been used to allow higher flows of hydrocarbons to the furnace lower cone, to extend the limits noted above, and to control the temperature of the burden. For cold discharge furnaces, some examples of techniques being used are cooling zone bleed and simplified center injection. However, prior art center injection techniques lack means to control or measure flow into the center injection system and lack means to force flow into the center injection line.
Another technique for temperature and carbon control which has been employed is the injecting of cold natural gas into the direct reduction furnace. The natural gas mixes with other gases already present in the furnace and is heated by the gas and solids already in the furnace. As the hydrocarbons in the natural gas are heated they crack to form H2 and deposit carbon on the product or they reform with H2O and CO2 in the gas furnace to make additional H2 and CO. The present limitation on the injection of natural gas is temperature. As more cold natural gas is injected, the center bed temperature decreases, which decreases the rate of reaction kinetics. At low flow rates of the cold natural gas, the production benefit from additional reducing gases will outweigh the disadvantage from decreased reaction kinetics. But when the temperatures in the center bed decrease to a certain point, any further production benefit from additional reducing gases will be negated by the decrease in reaction kinetics. This limits the amount of natural gas that can be added to the furnace for in situ cracking and reforming.
What is therefore needed is a means and method for increasing the amount of hydrocarbon gas supplied to the transition zone and/or cooling section of a direct reduction furnace while maintaining the temperature of the central reaction zone of the direct reduction furnace at a temperature favorable to the direct reduction of iron.
The present invention provides a method and apparatus which controls the temperature uniformity of the center portion of the iron burden in a direct reduction shaft furnace thereby allowing a higher amount of hydrocarbons to be used in the cooling zone. The invention is an efficient improvement of existing methods, particularly, the Midrex method and apparatus for direct reduction of iron which is incorporated by reference herein. Typically, the center portion of iron bearing material in the burden of a direct reduction furnace is cooler than the rest of the burden due to upflowing gases which is injected into the lower cooling section of the furnace and rises upwardly into the center portion of the reducing section of the furnace. By increasing the temperature of the burden in the center portion of the furnace, the iron is reduced under much more favorable conditions. Thus, the present invention is advantageous to achieve the objects stated herein.
Disclosed herein are methods for heating the center region of the furnace, particularly the burden. In a first embodiment of the invention, a hydrocarbon gas used in direct reduction may be preheated, which increases the temperature of the upflowing gas as it flows upwardly into the center of the burden. Alternatively, a portion of the upflowing gas may be removed before it enters the reduction zone of the furnace. The removed upflowing gas, known as hot bleed gas, may be ducted to a top gas scrubber of the furnace or may be mixed with the main reducing gas stream of the furnace for reintroduction to the furnace. Alternatively, hot reducing gas may be directly injected into the center portion of the burden, offsetting the effect of the upflowing gases. The center injected hot reducing gas may be split off from the main reducing gas stream or may be generated by a partial oxidation reactor. Finally, it will be appreciated by those skilled in the art that the above noted embodiment may be employed individually or in combination depending on the DRI plant facility.
The principal object of the present invention is to provide a method and apparatus for controlling temperature uniformity in the burden of a direct reduction shaft furnace.
It is another object of the present invention to provide an apparatus and method which allows the use of higher quantities of hydrocarbons or other gases within the lower portion of both hot and cold discharge furnaces so that product carbon and/or in-situ reforming may be increased while the temperature within the center of the direct reduction furnace is maintained at a temperature favorable to the direct reduction of iron, thereby increasing the degree of carburization of the iron product without adversely affecting the metallization of the iron burden.
Another object of the present invention is to provide an apparatus and method for conserving energy in a direct reduction furnace by maintaining a uniform temperature throughout the burden.
Another object of the invention is to provide a method and apparatus for increasing the temperature of the center burden of a direct reduction furnace in order to offset the cooling effect caused by upflowing gases into the reduction zone of the furnace, and thereby increasing metallization of the burden.
Another object of the present invention is to provide control options to DR plant operators to allow significantly higher levels of hydrocarbon gas additions, while reducing the temperature variations across the burden.